Many satellite communication systems have been designed, and some have been built and are operating. The ultimate goal of any of these systems is to provide the most efficient communication system with due regard for the format in which information is received for transmission purposes, as well as the capital expenditures required for construction of the system. In designing such a system there are, of course, many trade offs that can be made by varying different parameters such as the size of the antenna and the power which is to be transmitted. For instance, by increasing the size of the antenna and the power handling capacity of such antenna, the rate at which information is transmitted can be increased while at the same time maintaining the bit error rate. Of course, this increase in system capacity is paid for by the increased cost of building and operating the system. The first large scale communication system of this type which has been built and is operating employs the Intelsat satellite. In this system the antennas are relatively large (approximately 90 feet in diameter) and thus a reasonable bit error rate is maintained for the rather high transmission rate (64 megabits per second) of the system.
On the other hand, a second system which has been built and is operating employs the ANIK satellite (Telesat) in which the size of the antennas has been drastically reduced. In order to maintain the bit error rate at reasonable values (such as 1 .times. 10.sup.-6) for a transmission rate of 61 megabits per second, various well known techniques have been employed. One of these techniques is the use of forward acting error correcting coding networks on the transmit side, and corresponding decoding networks at the receive side. For example see Gabbard et al "The Telesat Common TDMA Terminal Equipment," Proceedings of the International Conference on Communications, San Francisco, June 1975.
The Telesat system employs TDMA techniques and thus it transmits different channels of data in a burst mode. This technique in turn, imposes certain requirements on the terminal. Thus, each different channel may be accepted by a different interface unit and each of the interface units may include a compression-expansion buffer memory (see U.S. Pat. No. 3,838,221) which operates to allow the burst mode transmission. More particularly, information may be received at a receive rate R.sub.s. A terminal may accept a plurality of different channels substantially simultaneously. Any terminal is allowed to transmit in a particularly determined time slot in a repetitive time frame. Each station's burst is divided into a plurality of sub-bursts, one for each of the different channels of information which is accepted for transmission purposes. Information which is received between bursts at any terminal is stored in the compression memory such that when that terminal's time slot occurs, the information can be transmitted at a relatively high transmission rate T.sub.s. Furthermore to transmit information from a plurality of channels each of the compression memories associated with different ones of the interface units is enabled for the sub-burst period associated with that interface unit to allow the information accumulated to be transmitted. On the receive side a complementary operation occurs where the information received is written into an expansion memory and the information is read out of the expansion memory at a slower rate than the information is received. In this fashion burst transmissions are enabled at the relatively high T.sub.s rate while the average rate of information transmission is equal to no more than the receive rate R.sub.s.
Due to the smaller antennas in the Telecast system forward acting error correcting coding has been employed. At the transmitting terminal this is implemented by employing a forward acting error correcting coding network on the transmit side of the compression-expansion memory. In this fashion, a single error correcting network operates on all transmitted data in any terminal and codes the information for transmission purposes. In a like fashion a decoding network is employed at the receive side of the receiving terminal such that the information received is decoded and then made available to the expansion memory in each of the different interface units.
Although this arrangement meets system requirements, i.e., it decreases the apparent bit error rate of the communication link to desired levels it does have a number of disadvantages. For once thing, the error correcting coding and decoding networks must operate at the relatively high transmit rate. Thus, the networks are relatively expensive. However, more importantly, the different formats which information is received for transmission purposes require different channel bit error rates for proper transmission. Thus, directly encoded PCM, whether speech or data may only require a bit error rate of 10.sup.-4. On the other hand for direct digital data channels a bit error rate of 10.sup.-7 is desired and for direct super group coding a bit error rate in the range of 10.sup.-6 to 10.sup.-7 is desired. The use of a single encoder-decoder pair operating on data received from the compression memory and providing data to the expansion memory must be designed to meet the most stringent requirements for the channels that the station is to handle. Clearly the apparent channel bit error rate will be less than required for some formats and, to that extent, the cost-effectiveness of the system can be improved.
It is therefore one object of the present invention to provide a terminal interface unit which allows the forward acting error correcting coding to provide a channel bit error rate which is selected to improve the uncoded channel bit error rate which is selected to improve the uncoded channel capacity to the extent required by the type of information which is to be transmitted. It is another object of the present invention to provide a terminal interface unit which includes forward acting error correcting coding operating at the relatively slower receiving rate R.sub.s rather than the relatively higher transmission rate T.sub.s.
It is another object of the present invention to provide such an interface for use with differentially encoded PSK by interleaving adjacent bits before encoding and de-interleaving to separate adjacent encoding caused errors for correction by the error correcting de-coding.